Droughts are a serious threat in this water-scarce state. The potential for more frequent and extreme droughts in the future will pose a major challenge to Arizona’s environmental, agricultural, and human systems. The frequency of wildfire occurrence and severity is projected to increase in Arizona.
Average annual temperature has increased about 2°F since the early 20th century, and recent upward trends in average temperatures and extreme heat are projected to continue in the future. Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century.
The summer monsoon rainfall, which provides much needed water for grazing lands and their ecosystems, varies greatly from year to year, and future trends are highly uncertain. Spring precipitation is projected to decrease, leading to negative impacts on late season snowpack accumulation and affecting valleys that rely on the melting snowpack for summer water supplies.
Arizona is the sixth largest state and its large geographic area encompasses diverse climates and a wide range in topography. The deserts in the southern portion of the state are some of the hottest and driest areas of the United States, while the higher terrain of the Colorado Plateau in the northeastern part of the state has a cooler climate, with cold winters and mild summers. In the mountain ranges, which run from the northwest to the southeast, the state experiences heavier precipitation and wide temperature variations. Average annual temperatures in the mountains are around 40°F at the highest elevations, while the southern portion of the state frequently experiences summer temperatures between 105°F and 115°F. Phoenix has the hottest climate of all major U.S. cities. The record high temperature for the state is 128°F recorded at Lake Havasu City on June 29, 1994, while the record low temperature was -40°F at Hawley Lake on January 7, 1971. The year 2014 was the hottest on record, with a statewide average annual temperature of 62.3°F, about 3°F above the long-term average.
The 21st century has been the warmest period on record for the state (Figure 1). Although Arizona has not experienced an increase in the number of extremely hot days (maximum temperature above 100°F; Figure 2a), the state has experienced an increase in the number of extremely warm nights (minimum temperature above 80°F; Figure 3) and a decrease in the number of very cold nights (minimum temperature below 0°F; Figure 4) since the 1970s. One notable trend is an increase in both daytime high and nighttime low summer temperatures, which has implications for heat waves in the state, which already experiences very hot conditions (Figure 2b).
Much of the state is characterized as arid to semi-arid with total annual precipitation ranging from less than 3 inches in the southwest to around 40 inches in the White Mountains in east central Arizona. Precipitation is highly variable from year to year, with statewide average precipitation ranging from a low of 6.04 total inches in 1956 to a high of 22.77 inches in 1905. The driest multi-year period occurred during the late 1890s/early 1900s and was immediately followed by the wettest period on record (Figure 2c). The driest 5-year period was 1899–1903 and the wettest was 1905–1909. The most recent two decades have also been relatively dry, with 14 of the last 20 years experiencing below average precipitation. Snowfall is rare in the southern desert region but does occur at the higher elevations where the snow can reach depths of more than 100 inches. Snowpack plays a critical role in supplying water for both urban and agricultural areas in the lower Salt River Valley and the lower Gila River Valley, and is critical to forest health and groundwater recharge across the entire state.
An important feature of Arizona’s summer climate is the North American Monsoon, which causes large amounts of rain to fall from early July to mid-September. Precipitation during the monsoon season is highly variable from year-to-year. During the first part of the 21st century, precipitation was below normal, while the most recent 5-year period (2010–2014) experienced above average precipitation due to warmer sea surface temperatures off the Pacific Coast and a very active hurricane season in 2014 (Figure 5). In the southernmost portion of the state, monsoon rainfall accounts for more than half of the annual precipitation and plays an important role in supporting agriculture and ecosystems. The monsoon rains are highly beneficial but can occasionally be destructive. On September 8, 2014, extremely heavy monsoon rain associated with a decaying east Pacific hurricane caused significant damage and flooding around the Phoenix area. The record for single-day rainfall was broken, with several stations reporting more than 4 inches of precipitation for the day.
The historical record indicates periodic occurrences of extended wet and dry periods (Figure 6). Currently, the state is approaching the 20th year of a long-term drought. Multi-year periods of high and low precipitation can cause very large variations in reservoir supplies. The latest western U.S. drought has resulted in record low levels of water in Lake Mead, which is a critical water resource for Arizona. Since reaching high levels in the 1990s, water levels have been falling, reaching historic lows in 2009 and again in 2014 (Figure 7). Long-term droughts also raise the risk of wildfires, already a concern for this arid state. In 2011, the Wallow Fire consumed more than 500,000 acres in eastern Arizona, making it the largest wildfire on record in the state.
Unlike many areas of the United States, Arizona and other southwestern states have not experienced an upward trend in the frequency of extreme precipitation events. The number of these extreme events (more than 1 inch of precipitation) has been variable throughout the entire period of record (Figure 2d). Since drought conditions began in the 2000s, the occurrence of these extreme precipitation events has been near to below normal, with the exception of the most recent 5-year period (2010–2014) when the state experienced an above average number of such events.
Under a higher emissions pathway, historically unprecedented warming is projected by the end of the 21st century (Figure 1). Even under a pathway of lower greenhouse gas emissions, average annual temperatures are projected to most likely exceed historical record levels by the middle of the 21st century. However, there is a large range of temperature increases under both pathways, and under the lower pathway, a few projections are only slightly warmer than historical records. Extreme heat is of particular concern for Phoenix and other urban areas where the urban heat island effect raises summer nighttime temperatures. This increase will cause future heat waves to become more intense, a concern for this region, which already experiences extremely hot conditions.
Although projections of overall annual precipitation are uncertain, spring precipitation is projected to decrease (Figure 8). This decrease will have negative impacts on late season snowpack accumulation in the mountains that feeds water supply reservoirs, reducing water flow to the valleys that rely on the snowpack for summer water supplies. Additionally, as temperatures increase, the average lowest elevation at which snow falls (the snow line) will also increase. This will increase the likelihood that precipitation will fall as rain instead of snow, reducing water storage in the snowpack, particularly at lower mountain elevations which are now on the margins of reliable snowpack accumulation. Higher spring temperatures will also result in earlier melting of the snowpack, further decreasing water resources needed for irrigation during the hot summer months.
Naturally occurring droughts are expected to become more intense during the cool season. Future projections of precipitation are uncertain, including those related to the North American Monsoon, however, as noted above, spring precipitation is likely to decrease. Even if precipitation does not decrease, higher temperatures will intensify naturally occurring droughts by increasing water evaporation. This will further reduce streamflow, soil moisture, and water supplies. Drought will not only further challenge limited agricultural resources but also increase the frequency of dust storms and the occurrence and severity of wildfires